cPnginx version 10.0 or higher only support cpanel paper lantern theme and its clones. It doesn’t support x3 or any other depreciated theme format. If you have a custom clone of paper_lantern theme you can integrate it to your theme as follows,
# cp -arf /usr/local/cpanel/base/frontend/paper_lantern/cpnginx
/usr/local/cpanel/base/frontend/YOUR_CUSTOM_THEME_PATH/